Generally, a polyester resin which is prepared from an aromatic and an aliphatic dicarboxylic acid and an alkylene glycol of appropriate structure has good physical and chemical properties. Also, it is widely used as textiles, films, adhesives and so on, because of its solubility in conventional solvent, flexibility, high adhesion to a variety of materials, coating workability, and so on.
In the preparation of polyester resins, the ratio of each raw material in the main chain of the final polyester resin is changed according to the reactivity of each raw material in case of an esterification reaction or an ester exchange reaction. While the ratio of each raw material in the main chain of the final polyester resin is changed according to the evaporation degree of each raw material in case of a polycondensation reaction. Among the diol component represented by alkylene glycol, the reactivity of a secondary or a tertiary diol is lower than a primary diol, and the reactivity of a tertiary diol is lower than a secondary diol, therefore the remaining rate of the secondary and the tertiary diol is low in the main chain of the polyester resin.
A polyester resin of high molecular weight, for example, polyethyleneterephthalate sometimes has unexpected colors by catalyst residues such as manganese, zinc, iron, and molybdenum which are produced from a preparation process of terephtalic acid. U.S. Pat. No. 5,674,801 discloses a method for preparing polyethyleneterephthalate using a polymerization catalyst containing cobalt salts which is combined with aluminum chloride, hydroxy aluminum, acetic acid aluminum or hydroxy aluminum chloride. The above stated patent states a method of preparing a colorless or white final product by blocking a metal catalyst residue using a phosphoric acid. The method can block the metal catalyst residue through forming a heteropolyacid by combining lots of metal with a phosphoric acid in the strong acidic medium. However, there are some disadvantages in that the very strong phosphoric acid is used.
A polyester resin is burnt easily in contact with flame, therefore the usage as the adhesives for electronics, flame retardant paints and so on which need flame resistance, has been limited. When the polyester resin is forced to be used, it should be mixed with a lot of flame retardant. As the flame retardant, inorganic flame retardants such as antimony and metal hydrate and halogenated organic flame retardants are generally used. But these flame retardants have problems in that they generate toxic gas in their combustion, as well as cause troubles to the insulation and the storage stability which are needed for the composition of adhesives for electronics or the coating agent.